Abstract: :
Purpose:To map and identify reversible nerve–fiber layer(NFL) swelling in ethambutol (EMB)–induced optic neuropathyusing optical coherence tomography (OCT). A serious complicationof ethambutol is a toxic mitochondrial optic neuropathy thatproduces impairment in visual acuity, contrast sensitivity,and color vision in as many as 6% of patients. OCT has shownNFL swellings in LHON, an irreversible genetic mitochondrialoptic neuropathy. However, deficits in EMB–induced opticneuropathy may be reversible.Methods: One subject with a history of EMB–induced opticneuropathy was administered a full neuro–ophthalmologicexamination including visual acuity, color vision, contrastsensitivity, fundus examination, visual fields testing, andOCT at three separate visits. These three visits occurred attwo, five, and eight months after EMB discontinuation. OCT wasperformed on both eyes of the subject using the retinal nervefiber layer (RNFL) analysis protocol on the three visits.Results: OCT quantified a significant increase in NFL thicknessin the inferior quadrant of the right eye (201µm thickness,or 298% increase in thickness) on the second visit than seenon the first visit (91µm). This corresponded well withthe presentation of new–onset right–hemisphere superiorvisual field defects in the subject’s right eye on thesame visit (MD = –9.0 +/– 6.24, p<.01). However,at the third visit, there was a notable decrease in the NFLthickness in the subject’s inferior quadrant (162 µm,or a 19% decrease in thickness) when compared to the secondvisit. Additionally, there was a concurrent improvement in thesubject’s superior visual field defect when compared tothe second visit (MD=–5.4 +/– 3.29, p<.01).Conclusions: We are able to identify and quantify, for the firsttime, reversible NFL changes in a patient with EMB–inducedoptic neuropathy using OCT. There is evidence of reversibleaxonal swelling in the peripheral RNFL, which is occupied byaxons of larger caliber than found in the macula. These axonscontain a larger percentage of magno–cellular fibers thatsubserve motion stereopsis, depth perception, and low–contrastsensitivity. These larger axons seem to have a greater capacityfor compensation for mitochondrial injury. OCT can be a valuabletool in the quantitative analysis of optic neuropathies.